1. Field of the Invention
The present invention relates, in general, to a device for inrush current prevention and dynamic braking in a motor, and more particularly, to a device for inrush current prevention and dynamic braking in a motor, wherein a dynamic braking circuit is constructed so as to jointly use a resistor and a relaying unit of an inrush current preventing circuit and a diode of an inverter.
2. Description of the Related Art
A three-phase motor refers to a motor having a coil wound in a triangle. A device for inrush current prevention and dynamic braking in the three-phase motor is used to prevent inrush current when a three-phase voltage required to drive the three-phase motor is generated and to perform a dynamic braking operation relative to the three-phase motor. As illustrated in FIG. 1, the device for inrush current prevention and dynamic braking in the three-phase motor comprises a diode rectifying unit 130 rectifying alternating current (AC) power supplied from an AC power supplying unit 120, which supplies commercial AC power (AC 110/220V), an inrush current preventing circuit 160 preventing inrush current when initial power is applied, a capacitor 140 smoothing the power rectified in the diode rectifying unit 130, an inverter 150 inverting direct current (DC) power from the capacitor 140 into the AC power having a variety of frequencies, and a dynamic braking circuit 170 short-circuiting a power inputting terminal 112 of the three-phase motor 110.
The inrush current preventing circuit 160 includes a first resistor 162 disposed between the AC power supplying unit 120 and the capacitor 140, an end thereof being connected to the capacitor, and a first relaying unit 164 which is turned off so as to allow a voltage rectified by the diode rectifying unit 130 to be transmitted to the capacitor 140, by passing through the first resistor 162 or the first relaying unit 164 is turned on so as to allow the voltage rectified by the diode rectifying unit 130 to be transmitted to the capacitor 140, without passing through the first resistance 162.
The inverter 150 comprises first, second and third inverting circuits 152, respectively, connected to the capacitor 140, in parallel, and each of the inverting circuits 152 comprises a pair of inverting elements 154 having a transistor 154b and a diode 154a connected to each other in parallel. Further, each power inputting terminal 112 of the three-phase motor 110 is connected at a common node between one of the pair of inverting elements 154 and the remaining one of the pair of inverting elements 154 of the respective first, second and third inverting circuits 152, such that a three-phase voltage is inputted from the inverter 150.
In the dynamic braking circuit 170, a pair of dynamic braking diodes 176 are connected to each power inputting terminal 112 of the three-phase motor 110, one of the pair of dynamic braking diodes 176 is in a forward direction and the other one of the pair of dynamic braking diodes 176 is in a reverse direction. The pair of dynamic braking diodes 176 connected to each other in the forward direction are connected to a second resistance 172 and a second relaying unit 174. In the dynamic braking circuit 170, the three-phase motor 110 is allowed to stop suddenly while the three-phase motor 110 is in operation, thereby preventing the three-phase motor 110 from being forcibly rotated due to an external force after the three-phase motor 110 has stopped. Further, the second relaying unit 174 remains turned-off while the three-phase motor 110 is driving, and is turned-on while the three-phase motor 110 is in suspension or after the three-phase motor 110 stops, thereby allowing the three-phase motor 110 to stop suddenly or restraining the three-phase motor 110 from rotating due to the external force.
However, in a conventional inrush current prevention and dynamic braking device in a motor, the operation of the inrush current preventing circuit 160 is performed only when the power is initially supplied to a system such as a motor control system and is no longer needed in operating a circuit after the power is supplied and the capacitor 140 is fully charged.
Further, the operation of a conventional dynamic braking circuit 170 is performed only while the three-phase motor 110 is in suspension or after the three-phase motor 110 is stopped, and is no longer needed in operating the circuit after the power is supplied and the capacitor 140 is fully charged, that is, while the three-phase motor 110 is in normal operation.
However, to prevent an initial overcurrent from the three-phase motor 110, a resistor of a large capacity or a thermistor is generally used as the first resistor 162 in the inrush current preventing circuit 160. Further, in a case of the dynamic braking circuit 170, a resistor of a large capacity or a thermistor is used as the second resistor 172 so as to prevent the three-phase motor 110 from being damaged due to a large current flowing in the wound coil of the three-phase motor 110 produced when the three-phase motor 110 is rotated due to an external force. Thus, a dimension of a product is increased with respect to the circuit, and a number of components thereof is increased because of the first and second resistors 162 and 172, the first and second relaying units 164 and 174, and the dynamic braking diodes 176 comprising the inrush current preventing circuit 160 and the dynamic braking circuit 170, thereby increasing a production cost.